In an attempt to improve the quality of the air that we breathe, there has been an increased effort to reduce air pollution due to vehicle emissions. One solution to this growing problem is on-road testing of in-use vehicles using Remote Sensing Devices (RSD). RSD containing infrared and/or ultraviolet radiation detectors are generally known. As vehicles drive by, these RSD measure and record the vehicles' exhaust emissions and other important information.
For example, one known compact, portable RSD comprises an infrared (IR) beam emitter, detectors and a microprocessor. In operation, the RSD measures the concentration of hydrocarbon (HC), carbon monoxide (CO), and carbon dioxide (CO.sub.2) within a vehicle's emission. The RSD determines the vehicle's emission concentration by passing a beam of infrared radiation through the vehicle's exhaust as the vehicle travels by the RSD positioned alongside the highway. The IR emitter transmits a beam of IR energy along a path which crosses the traffic lane and encounters a reflector (a mirror) which returns the beam through one or more radiation filters to one or more detectors which are positioned next to the emitter. The IR emitter is located on one side of the vehicle's path (such as a road or highway ramp) at a proper height so that the beam intersects the vehicle's exhaust plume. The IR emitter sends the IR beam into the detectors on a continuous basis. The radiation filters selectively permit radiation of certain characteristic wavelengths to pass through to the detectors while filtering out radiation of other wavelengths. The detectors operate to convert the infrared energy reflected from the reflector(s) to an electrical signal which is measured in voltage and is continuously transferred to the computer where this information is used to compute and produce output signals indicative of the percentage of CO, CO.sub.2, and HC in the vehicle's emission.
In order to ensure the accuracy of the exhaust readings, the RSD must be initially calibrated with a gas having known concentrations of CO, CO.sub.2, and HC under known atmospheric conditions prior to the vehicle's approach. When there is no vehicle emission in the path, such as during calibration, the detector emits a strong electrical signal because the greater the infrared energy received at the detector, the stronger the electrical signal produced by the detector. However, if there is a presence of CO, CO.sub.2, or HC in the infrared light path, these constituents will absorb a portion of the infrared radiation at wavelengths characteristic of each of these emission components. The more CO, CO.sub.2, or HC in the exhaust gas; the more radiation at each characteristic wavelength is absorbed, and thus the less radiation sensed by the detectors at these wavelengths. Consequently, the less radiation detected, the lower the level of electrical signal which is generated by the detector. Therefore, the electrical signals emitted by the detectors in response to radiation having the characteristic wavelengths are proportional to the concentration various components of the vehicle's emission.
When a vehicle interrupts the light beam of the RSD, the RSD stores the current readings of the ambient conditions, performs electronic calibration, and starts to monitor the vehicle's emissions. When the exhaust has been sampled, the results are compared to the carbon monoxide, carbon dioxide, hydrocarbon, and nitrogen oxide levels recorded during calibration are stored in the computer memory.
In calculating the concentrations of various components of the vehicle emissions, the computer algorithm relies on detectors coefficients. Emission results are obtained, by computing the ratios of the CO, CO.sub.2, and HC voltages to the reference voltages, applying unique detector coefficient, and resealing these arbitrary units into calibrated CO, CO.sub.2, and HC values using calibration curves determined in a laboratory utilizing special flow cells with a controlled mixture of CO, CO.sub.2, and HC. Those data are then analyzed by the computer by a least squares procedure to determine the amount of each gas constituent within the exhaust emission.
In order for the RSD to accurately determine the concentration of gas, the radiation filters must operate to selectively pass the desired wavelength through to the detector in an efficient and reliable manner. Radiation filters are used to select wavelengths of radiation. A radiation filter works by excluding all but a limited band of radiation centered about a specific wavelength. A radiation filter is a two-port network that passes signals with wavelengths within a specified band (the filter passband) and attenuates signals whose wavelengths lie outside this band (in the filter stopband). The center wavelength is the wavelength that allows the most radiation through the filter within the range of wavelengths that will be passed through.
CO, CO.sub.2, and HC each have different characteristic wavelengths because each constituent absorbs infrared energy at different wavelengths. The amount of energy that reaches each detector is critical to the RSD's ability to accurately measure the various gas constituents since the detectors which are employed in the RSD generate an electrical signal proportional to the amount of radiation which impinges on the detector. Thus, it is essential to a system's detection of a vehicle's emission to select radiation filters with the proper center wavelengths and bandwidths to ensure accurate concentration readings.
Existing devices exhibit numerous drawbacks. For example, improper selection of a RSD's radiation filter can generate contamination in the detector's reading which can adversely affect the system's reliability. Contamination occurs when the radiation filter fails to shield out radiation which may be absorbed by other components of the vehicle emission such as water, pollutants, or dust. This radiation may contaminate the reading received at the detector thereby reducing the accuracy of the measurement of the desired constituent within the exhaust emission.
In response to these concerns, RSD have been developed to provide remote emissions testing of vehicles while in use. RSD employ radiation passed through the vehicle exhaust plume to detect and determine the concentration of one or more constituents of the vehicle emission.
These and other drawbacks exist.